For example, as a reversible electromagnetic contactor which is connected to a feeder circuit of an induction motor to control a forward/reverse operation of the induction motor, there is known a device in which a reversible unit is attached by being mounted over two adjacently disposed electromagnetic contactors, and the reversible unit mechanically locks a simultaneous turn-on operation (simultaneous ON operation) of the two electromagnetic contactors (e.g., Patent Document 1).
Each of FIGS. 9 and 10 shows a conventional reversible electromagnetic contactor similar to Patent Document 1.
As shown in FIG. 9, in two adjacently disposed electromagnetic contactors 1a and 1b, display windows 3a and 3b are provided in arc extinguishing covers 2a and 2b, and operation display pieces 4a and 4b protrude in the display windows 3a and 3b from internal portions of the devices.
As shown in FIG. 10, the operation display piece 4a of one of the electromagnetic contactors 1a is a member which is fixed to a movable contact support 4 disposed inside the electromagnetic contactor 1a and extends into the display window 3a, and the operation display piece 4b of the other electromagnetic contactor 1b is also a member which is fixed to a movable contact support disposed inside the electromagnetic contactor 1b and extends into the display window 3b, though not shown.
As shown in FIG. 9, a reversible unit 6 has a unit bottom plate 6a which is connected to the arc extinguishing covers 2a and 2b by being mounted over the two electromagnetic contactors 1a and 1b, and a unit cover 6b which has a peripheral edge portion to be engaged with that of the unit bottom plate 6a, and a first interlock plate 6c, a second interlock plate 6d, and a lock piece 6e which constitute a lock mechanism are mutually rotatably connected to each other in the internal space of the unit bottom plate 6a and the unit cover 6b. On the back surfaces on end portion sides of the first and second interlock plates 6c and 6d, there are formed tubular connection pieces 6f and 6g which protrude to the outside through opening portions 6h and 6i formed in the bottom plate 6a. In addition, on the front surfaces on the end portion sides of the first and second interlock plates 6c and 6d, there are formed reversible unit operation display pieces 6k and 6m which protrude into a pair of display windows 6n and 6p formed in the unit cover 6b. 
A plurality of hook portions 6j is formed on the unit bottom plate 6a, a plurality of engagement holes 2c corresponding to the individual hook portions 6j is also formed in the arc extinguishing covers 2a and 2b, and the plurality of hook portions 6j is detachably engaged with the corresponding engagement holes 2c when the unit bottom plate 6a contacts with the arc extinguishing covers 2a and 2b. 
The connection pieces 6f and 6g of the reversible unit 6 are connected to the head portions of the operation display pieces 4a and 4b of the two electromagnetic contactors 1a and 1b in fitting relation, the lock piece 6e rotates one of the first and second interlock plates 6c and 6d while regulating the rotation of the other interlock plate, and only one of the operation display pieces 4a and 4b is allowed to move, whereby a simultaneous turn-on operation of the two electromagnetic contactors 1a and 1b is mechanically locked.
Patent Document 1: Japanese Patent Application Publication No. H3-266325